This invention is related to switching amplifiers for inductive loads and is an improvement to the amplifiers in my U.S. Pat. Nos. 4,100,471 and 4,140,956. In particular, this invention is a frequency stabilizer and load fault detector for dual threshold switching amplifiers. This invention is also an improvement to my copending application "Switching Transconductance Amplifier for Inductive Loads", Ser. No. 89,678.
Stepping motors are a type of synchronous motor which may be driven over a wide range of speeds including zero. The low frequency energization of stepping motors must be current limited to keep the motor from incineration. The prior art contains many approaches to limiting the motor current. Of these, switching amplifiers provide the greatest efficiency. The switching amplifier disclosed herein is the single voltage type and not the dual voltage variety which uses the higher voltage to rapidly reach the desired current level and a lower voltage to sustain the current and to maintain a motor position without excessive motor temperature rise.
Even among the single voltage switching amplifiers there are different designs. One technique allows the inductive current to rise rapidly to a predetermined level after which the current is allowed to delay until the end of a fixed frequency clock period. Alternately, upon reaching the predetermined level, the current is allowed to decay for a fixed period. Both of these designs employ a fixed timing. Although fixed timing techniques limit amplifier dissipation and audible noises from the motor, fixed timing techniques have poor high speed performance when used in fractional stepping or microstepping motor drives. The operation of stepping motors at elevated stepping rates have often suffered from mid-frequency resonance. Mid-frequency resonance is the rotor oscillation whose frequency is unrelated to any motor drive frequency. Mid-frequency resonance is created by the reduction of the stabilizing damping forces. Although this occurs only in voltage driven motors, stepping motors exhibit this phenomenon even with current drives at high speeds because at high speeds the current amplifiers are no longer compliant and exhibit the voltage drive characteristic which creates mid-frequency resonance.
The dual threshold switching amplifier of my U.S. Pat. No. 4,140,956 depends upon the inductance of the load and the voltage of the power supply to determine the switching frequency. If the motor is shorted the inductance is drastically lowered and the switching frequency rises significantly. The high frequency causes the amplifier to dissipate excessively and to fail. A wiring short may also shunt current away from the amplifier current feedback making it switch more rapidly, often with the same failures.
The amplifier of my U.S. Pat. No. 4,140,956 has a ground-referenced input which is subject to ground current induced noise. This is a disadvantage because ground current noise creates spurious oscillations in the switching amplifier.